Alloy steels



2,693,412 P aterited Nov. 2, 1954 2,693,412 ALLOY STEELS John Glen, Glasgow, Scotland, assignor to Colvilles Limited, Glasgow, Scotland, a British company No Drawing. Application November 3, 1951, Serial No. 254,798

Claims priority, application Great Britain November 10, 1950 '7 Claims. (Cl. 75-423) This invention relates to low carbon alloy steels, its object being to provide easily weldable steels particularly suitable for surface 'protection by methods such as chromising and with high creep resistance at elevated temperatures. Chromising is defined as the diffusion of chromium into the surface of a metallic article by any method in which the article is brought into intimate contact with chromium or a fluid chromium compound at an elevated temperature. The steels provided under the invention can be easily fabricated into superheater tubes and other articles.

The low alloy steels hitherto in general use for high temperature applications such, for example, as for boiler or superheater tubes and steam pipes have been found to be unsuitable when the metal temperature exceeds about 1050 F., in some cases because of insufficient resistance to scaling and in other cases because of low creep strength. This invention provides a material which by virtue of its high creep resistance is capable of withstanding service stresses at temperatures up to about 1150" F., and which is rendered resistant to scaling by chromising or some other method of surface protection.

In accordance with the present invention we provide an alloy steel containing:

From 0.03 to 0.2 per cent carbon From 0.05 to 0.4 per cent vanadium From 0.10 to 1.0 per cent molybdenum From 0.3 to 1.0 per cent manganese From 0.05 to 0.5 per cent titanium together with incidental ingredients and impurities in ac cordance with common steel-making practice, the balance consisting of iron. Incidental ingredients are those which, at the option of the manufacturer may or may not be introduced in the process of manufacture and include, for example, silicon. The main impurities are commonly sulphur and phosphorus.

The preferred composition of the alloy is as follows:

Percent Carbon -L .10 to .15 Vanadium .2 to .3 Molybdenum .5 to .6 Manganese .5 to .7 Titanium .2 to .3 Silicon .1 to .5

the balance being made up of iron and the usual impurities. Moreover the ratio of titanium to carbon is preferably between 2 and 3.

In further accordance with the present invention, steel articles fabricated from the low-carbon alloy steel provided in the present invention are treated on the surface by chromising.

For example, in accordance with the present invention, we provide superheater tubes manufactured from the low-carbon steel and chromised inside and outside, the chromised surface resisting attack by steam on the inside of the tubes and by oxygen and other gases on the outside of the tubes.

As a result of the present invention there is obtained:

(a) An easily weldable ferritic steel with high resistance to creep at elevated temperatures suitable for the manufacture of tubes, pipes and other components for use 'at high temperatures.

(12) A high tensile ferritic steel which has exceptional carbide stability at elevated temperatures and which is therefore eminently suitable for chromising to protect the surface from oxidation, scaling or corrosion.

(c) An article fabricated of steel with high creep resistance and with the surface protected from scaling and corrosion by a method such as chromising.

The following examples of steels in accordance with the present mvention are given:

and either air cooled, oil quenched or water quenched, 1

depending on the thickness of the section and the composition of the steel. It has been found that this high temperature treatment takes the carbides properly into solution.

It has also been found that in order to obtain the proper precipitation of the desired carbides to give the maximum creep resistance, the steels should be thereafter given one or more tempering treatments at temperatures up to 800 C., from which temperature they are either air or furnace cooled depending on the thickness of the section and the composition of the steel. After this treatment the steels have a hardness of approximately 200 to 300 Brinell.

The steels, after the aforementioned heat treatment, provide material having a relatively high tensile strength and good ductility at normal temperatures, together with high creep resistance at elevated temperatures up to about 1150 F.

More specifically the heat-treatment is a complex one, comprising heating up to at least 1050 C., cooling to atmospheric temperature at a rate (a) controlled to precipitate carbon as carbides in a finely dispersed form, thereafter heating to a temperature below the austenitic change point in order to produce complex carbides con-- taining titanium, and allowing to cool again to atmospheric temperature at a rate (b) controlled so that further precipitation of carbides can occur during cooling.

The austenitic change point is normally not higher than about 850 C.

Rate (a) may be effected, for example, by air cooling, oil quenching, or water quenching. The rate is influenced by the shape and size of'the article; the larger the cross section of the article, the slower the resultant cooling rate by any of these methods.

For the same sizeof article air cooling gives a relatively slow rate, oil a faster rate, and water a faster rate still. Thus to obtain the desired crystalline structure of the metal, small articles may be air-cooled, but large articles may require to be quenched in oil or water.

Rate (b) may be effected, for'example, by air or furnace cooling; if the article is of large cross-section air cooling may be satisfactory, but if it is of small cross-section the cooling rate may be retarded by some method such as furnace cooling.

For long time service at temperatures above about 1050 F. surface protection is necessary to prevent scaling. Such surface protection may be obtained by forming on the surface an inert and impervious coating. A particular method of surface protection for which the steel is eminently suitable is the formation of a chromium-rich surface layer by the chromising process.

It is known that the phenomenon of carbon migration is one of the most serious factors to be overcome in steels suitable for chromising. Hitherto this has been overcome by the additionof titanium to the steels, the pres- O, Si, Mn, Mo, V, Ti, S, P, Per- Per- Per- Per- Per- Per- Per- Percent cent cent cent cent cent cent cent ence of which element increases the thermal stability of the carbides, but such titanium-bearing steels hitherto developed have a relatively low tensile strength with relatively poor creep resisting properties at elevated temperatures.

The steels covered by this invention are characterised by exceptional carbide stability due to the presence of carbon, molybdenum, vanadium and titanium in controlled proportions, and are thus suitable for chromising. Where chromising has been employed, the aforementioned heat treatment is carried out after the chromising process.

After chromising and the aforementioned heat treatment, the steels have the same high tensile strength, and the creep properties are identical to those of the steel without chromising, with the added advantage that the presence of the chromium-rich layer on the surface resists scaling and thus substantially delays the onset of interorystalline fracture, which is the normal mode of failure of these types of steels at high temperatures. Without the protection of the chromised surface, oxidation occurs and intercrystalline cracking at the surface may occur at a comparatively early stage. Tests to fracture have been carried out under creep conditions and show that as the time to fracture is increased to very long'periods the life of the chromised steel becomes many times greater than that of the unchromised steel.

Under the invention, therefore, a steel has been developed which after heat treatment has substantially better creep properties than existing ferriticsteels. At the same time, because of the chromised surface, scaling at temperatures up to 850 C., is not progressive; as soon as the very thin film forms on the surface further scaling ceases.

As the steel can be softened readily to a hardness of approximately 120 Brinell, fabrication of this material is almost as easy as that of ordinary mild steel.

Furthermore, since the steels can be easily heat treated to give a minimum tensile strength of 40 tons/sq. inch, they widen the field, of application of chromised articles.

The standard steels at present in general use for such applications as boiler superheater tubes and steam pipes are the 0.5% molybdenum steel and the 1% chrome 0.5% molybdenum steel. The latter steel has been proposed for superheater tubes where the metal temperature is 1050 F.

The creep properties reported in published literature for a 1% chrome 0.5 molybdenum steel very considerably. Based on a creep rate of one ten-millionth of an inch per inch per hour after expiry of 1000 hours (1% in 100,000 hours) the highest values of stress reported are as follows:

Tons/ sq. inch 6.2

950 F 1000" F 4.6 1050" F 3.0

whereas stress values for the low-carbon low-alloy steel in accordance with this invention have been found as follows:

1050 F 8.0 tons/sq. inch or more. 1100 F 6.0 tons/sq. inch. 1150 F 4.0 tons/sq. inch.

Limit of proportionality (tons/ sq. inch) 18.5 .05% proof stress'(tone/sq. inch) 28.4 .2% proof stress (tons/sq. inch) 31.0 .5 proof stress (tons/sq. inch) 32.0 Ultimate tensile strength (tons/ sq. inch) 40.8

Elongation on gauge length of four times the square root of the cross-sectional area of the specimen,

per cent Reduction of area, per cent 69 The following example of a practical method of carrying out the invention is described below:

(1) Creep test pieces 0.357 inch diameter were manufactured from steel in accordance with the invention having the particular analysis carbon 0.125%, silicon 0.45%, manganese 0.47%, molybdenum 0.52%, vanadium 0.305%, titanium 0.195%, the balance being iron and impurities.

These test pieces were chromised in the following manner: They were packed in a gas-tight retort and completely surrounded with a mixture of pieces of ferrochromium and of sillimenite a porous refractory ceramic material, which had been previously impregnated with chromous chloride. All traces of oxygen were swept from the retort by introducing a stream of hydrogen, and the retort was then heated in a gas-fired furnace. A hydrogen atmosphere was maintained in the retort throughout the heating cycle. When the temperature inside the retort was 1080 C., a stream of anhydrous hydrogen chloride gas was introduced into it for approximately fifteen minutes in order to provide, by reaction with the ferro-chromium, a surplus of chromous chloride. The retort was maintained at this temperature for eight hours, after which it was allowed to cool to normal temperature and the test pieces withdrawn.

Scaling tests in air at 850 C. indicated that there was virtually no scaling. After a thin film of oxide formed on the surface, no further scaling occurred.

Thereafter the chromised test pieces were heat treated by heating to 1150 C. for half an hour, followed by air cooling. Precautions may be necessary to avoid oxidation of the chromised coating during this treatment; for example, the article may be coated with aluminium or aluminium paint. They were then tempered at 750 C. for two hours and air cooled. The following tensile properties were obtained:

0.5 proof stress, tons/sq. inch 33.4 Ultimate tensile strength, tons/ sq. inch 42.5 Elongation on gauge length of four times the square root of the cross-sectional area of the specimen,

per cent Reduction of area, per cent 67 A creep test at a stress of 6 tons/sq. inch and a temperature of 600 C. gave the following creep rates:

Time Creep Rate 500 Hours 2.35 10' iuch/ineh/hour. 1,000 Hours 1.25X10' inch/inch/hour 1,500 Hours 5.8)(10' inch/inch/hour.

(2) 4 inch diameter billets were hot rolled from a 2 /2-ton electric furnace ingot having the following composition:

A billet was rough-turned to 3% inch diameter, pierced and hot rolled to a tube in a pilger mill. This tube was then cold drawn to the finished dimensions which were 1% inch outside diameter by 3 standard wire gauge. A length of this tube was chromised as described above. Thereafter, the chromised tube was heat treated by heating to l/1140 C. for one-half hour and cooling in air. t was then tempered at 750 C. for 4 hours and cooled in the furnace. In this condition the material had a Brinell hardness of 190. A length of this tube was then bent cold round a diameter of 12 inches. A careful examination showed that the bent tube was free from defects and the chromised coating was unimpaired.

I claim:

1. An easily weldable ferritic steel with high resistance to creep at elevated temperatures approximating 1150 F. suitable for the manufacture of tubes, pipes and other components for use at high temperatures, and consistmg of:

2. A ferritic steel as claimed in claim 1, in which the ratio of titanium to carbon is from two to three.

3. An easily weldable ferritic steel with high resistance to creep at elevated temperatures approximating 1150 F., suitable for the manufacture of tubes, pipes and other components for use at high temperatures, and characterised by exceptional carbide stability rendering it suitable for chromising, said steel consisting of:

Carbon .125 Vanadium .305 Molybdenum .52 Manganese .47 Titanium .195 Silicon .45 Sulphur .036 Phosphorus .012

the balance consisting of iron and incidental impurities.

4. An easily weldable ferritic steel with high resistance to creep at elevated temperatures approximating 1150 F., suitable for the manufacture of tubes, pipes and other components for use at high temperatures, and characterised by exceptional carbide stability rendering it suitable the balance consisting of iron and incidental impurities.

5. An easily weldable ferritic steel with high resistance to creep at elevated temperatures approximating 1150 F., suitable for the manufacture of tubes, pipes and other components for use at high temperatures, and characterised by exceptional carbide stability rendering it suitable for chromising, said steel consisting of:

Carbon .185 Vanadium .26 Molybdenum .53 Manganese .64 Titanium .43 Silicon .22 Sulphur .029 Phosphorus .022

the balance consisting of iron and incidental impurities.

6. An easily weldable ferritic steel with high resistance to creep at elevated temperatures approximating 1150" F., suitable for the manufacture of tubes, pipes and other components for use at high temperatures, and characterised by exceptional carbide stability rendering it suitable for chromising, said steel consisting of:

Carbon .13 Vanadium .30 Molybdenum .59 Manganese .73 Titanium .25 Silicon .18 Sulphur .037 Phosphorus .033

the balance consisting of iron and incidental impurities.

7. An easily weldable ferritic steel with high resistance to creep at elevated temperatures approximating 1150 F., suitable for the manufacture of tubes, pipes and other components for use at high temperatures, and characterised by exceptional carbide stability rendering it suitable for chromising, said steel consisting of between 0.1% and 0.2% carbon, between 0.1% and 0.4% vanadium, be tween 0.4% and 0.8% molybdenum, between 0.45% and 0.75% manganese, between 0.1% and 0.5% silicon, and a titanium content between one and three times that of the carbon content, the balance being made up from iron and incidental impurities.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,661,176 ink Mar. 6, 1928 1,817,888 Lowe Aug. 4, 1931 2,325,113 Craig July 27, 1943 2,370,179 McCarroll et al. Feb. 27, 1945 2,383,299 7 Endress Aug. 21, 1945 2,480,151 Malcolm Aug. 30, 1949 2,501,262 Carhart et al. Mar. 21, 1950 FOREIGN PATENTS Number Country Date 494,841 Great Britain Nov. 1, 1938 OTHER REFERENCES Titanium in Steel, pages 158, 159 and 161. Edited by Comstock et al. Published in 1949 by the Pitman Publishing Co., New York. 

1. AN EASILY WELDABLE FERRITIC STEEL WITH HIGH RESISTANCE TO CREEP AT ELEVATED TEMPERATURES APPROXIMATING 1150* F. SUITABLE FOR THE MANUFACTURE OF TUBES, PIPES AND OTHER COMPONENTS FOR USE AT HIGH TEMPERATURES, AND CONSISTING OF: THE BALANCE MADE UP FROM IRON AND INCIDENTAL INPURITIES. 